1. Field of the Invention
The present invention relates to a process for the growth of single crystals from semiconductor material by the Czochralski method.
2. The Prior Art
In this method, a cylindrical single crystal grows on a seed crystal, the seed crystal first being immersed in a melt and then drawn away from the surface of the melt. The melt is held in a crucible which consists of a quartz glass crucible and of a support crucible which is usually made of graphite and in which the quartz glass crucible is placed. The crucible rests on a rotatable and axially displaceable shaft and is surrounded laterally by resistance heating elements which generate sufficient heat radiation to melt the solid semiconductor material therein and to keep it in the molten state. In addition to semiconductor material, the melt contains, if required, added dopants and unavoidable amounts of oxygen and other impurities, which are dissolved out of the crucible wall. As a rule, the crucible and the growing single crystal are rotated in opposite directions because this makes it possible to influence to a certain degree the quantitative incorporation of oxygen and the distribution of oxygen and dopants in the melt and in the single crystal.
The melt continuously loses heat mainly via its free surface, the growing single crystal, the bottom of the support crucible and the shaft. This heat loss must be compensated by increasing the heating power of the resistance heating elements. The increased heat supply heats the crucible wall to well above the melting point of the semiconductor material. This is a problem with adverse consequences which are evident in particular in the growth of single crystals having diameters of more than 200 mm, because high heat outputs are required for the growth of such single crystals, due to the correspondingly large crucibles and melt volumes. One consequence is that, due to the high temperatures at the crucible wall, oxygen and other impurities are dissolved to a greater extent out of the crucible material and enter the melt. Another disadvantageous consequence is that island-like cristobalite layers form to an increasing extent on the crucible wall as a result of the overheating of the crucible. As a result of pitting corrosion, these layers may release particles which reach the crystallization boundary with a certain probability via convection flows and can terminate the dislocation-free growth of the single crystal. For this reason, the achievable dislocation-free growth in length of single crystals having large diameters has also been very limited and the ratio of length to diameter in the case of such single crystals is small. A further disadvantageous consequence of too high a temperature at the crucible wall results from the associated large temperature difference between the crucible and the crystallization boundary. It causes the occurrence of powerful uncontrolled material flows in the melt as a result of thermal convection, which lead to local fluctuations in the temperature and in the concentration of dopant and impurities. This situation is of course also reflected in the growing single crystal by an undesirable, locally fluctuating distribution of the dopants and of the impurities, in particular of the oxygen.
In addition, the temperature fluctuations in the melt increase the temperature-related stresses to which the single crystal is exposed during growth. There is an increasing danger that dislocations will form in the region of the crystallization boundary and may spread into the dislocation-free part of the single crystal to a length corresponding to the diameter of the single crystal. Because regions of the crystal which contain dislocations are not suitable for use as a starting material for electronic components, drastic reductions in yield are to be expected particularly in the case of large single crystals where the ratio of length to diameter is, for the stated reasons, in any case small.
For example, DD-270 728 A1 discloses that static magnetic fields superposed on the melt influence material flows and temperature distribution in the melt. Their effect consists in particular in the damping of convective material flows. At suitable field strengths, local temperature fluctuations are reduced and the incorporation of dopant in the single crystal is more homogeneous. However, the magnets required for generating magnetic fields having suitable field strengths are complicated in design and their energy requirement is relatively high. The above-mentioned publication and U.S. Pat. No. 5,178,720 therefore give preference to processes which operate with rotating magnetic fields. According to the process disclosed in U.S. Pat. No. 5,178,720, the incorporation of oxygen into the growing single crystal is controlled on the basis of a sequence according to which certain rotational speeds of single crystal and crucible are to be observed and the intensity of a certain component of the magnetic field is reduced with increasing crystal volume.
In spite of some improvements through the use of static and rotating magnetic fields in the growth of single crystals by the Czochralski method, the situation remains unsatisfactory. In particular, the known processes do not provide an adequate solution to the problems described, which have to be overcome in the growth of single crystals having diameters of more than 200 mm.